The invention relates to a method for optically monitoring a running fiber strand made of natural fibers, and more particularly to such a method in which at least one light signal is transmitted onto the fiber strand and in which a light signal emitted by the fiber strand is received by a detector and is evaluated to determine a foreign substance made of synthetic material. The invention relates further to a device for carrying out such a method. A fiber strand in the context of the present invention is taken to mean a fiber band or a roving which are subsequently further processed to form a yarn, or else the yarn itself.
A generic method and a generic device are known, for example from European Patent Publication EP 0 643 294 A1.
During further processing of fiber materials, it is known that the presented fiber material may contain impurities in the form of foreign substances and foreign fibers, which can lead to undesired irregularities in the yarn produced therefrom and therefore also in the textiles produced in the subsequent processes. To avoid impurities of this type arriving in the yarn or the textile, the fiber strand is optically monitored. A method and a device are known for this purpose from European Patent Publication EP 0 643 294 A1, in which the fiber strand is acted upon by a light signal. The reflection signal generated by the fiber strand is guided to an image by means of a detector. The luminous intensity of the image is compared with a predetermined threshold value by means of evaluation electronics. The predetermined threshold value thus provides the limit value for a still acceptable impurity in the fiber strand. If this limit value is exceeded, a fault signal is generated in order to trigger a process intervention.
The known method and the known device are based on the fact that the foreign substances and foreign fibers are present with regard to their appearance in such a way that they have a clearly different reflection behavior of light signals compared to the fiber to be produced. In the cases in which, for example, foreign fibers or foreign substances of the same color are incorporated in the fiber strand, identification is not possible with the known methods and the known device.